An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. II. Reappearance of morphologically normal synaptic contacts

Cholecystokinin expression after hippocampal deafferentiation: molecular evidence revealed by differential display-reverse transcription-polymerase chain reaction

The cortical information flow via the perforant path represents a major excitatory projection to the hippocampus. Lesioning this projection leads to massive degeneration and subsequently to reorganization in its termination zones as well as in primary non-affected subfields of the hippocampus. The molecular mechanisms and factors which are involved in the postlesional events are poorly defined. Using a differential display reverse transcription-polymerase chain reaction (DDRT-PCR) strategy, we located one band which occurred only in control hippocampus lanes and almost disappeared in the lanes of lesioned hippocampi. By sequencing, we identified the corresponding gene as cholecystokinin (CCK). Northern blot analysis confirmed a decreased transcription of CCK after lesion. In situ hybridization analysis was performed for localization and quantification of altered CCK transcription. We noted a significant downregulation of CCK transcription in the hippocampus (20%) and in the contralateral cortex (12%) 1-day after lesion (dal) and an increased signal in the ipsilateral cortex (10.5%). This pattern was altered, showing upregulation of CCK mRNA expression, reaching its highest level of 70% above control levels at 5 dal. In the hippocampus, the control level was reached again at 21 dal, whereas the cortex reached the control level at 10 dal. In comparison, the mRNA transcripts of the receptors CCK(A) and CCK(B) remained unchanged. Since CCK-containing neurons are involved in the modulation of pyramidal and granule cell excitability, our data indicate a time course correlation between CCK mRNA expression and postlesional axonal sprouting response in the hippocampus.

Visualizing changes in circuit activity resulting from denervation and reinnervation using immediate early gene expression

We describe a novel strategy to evaluate circuit function after brain injury that takes advantage of experience-dependent immediate early gene (IEG) expression. When normal rats undergo training or are exposed to a novel environment, there is a strong induction of IEG expression in forebrain regions, including the hippocampus. This gene induction identifies the neurons that are engaged during the experience. Here, we demonstrate that experience-dependent IEG induction is diminished after brain injury in young adult rats (120-200 gm), specifically after unilateral lesions of the entorhinal cortex (EC), and then recovers with a time course consistent with reinnervation. In situ hybridization techniques were used to assess the expression of the activity-regulated cytoskeleton-associated protein Arc at various times after the lesion (4, 8, 12, 16, or 30 d). One group of rats was allowed to explore a complex novel environment for 1 hr; control operated animals remained in their home cage. In unoperated animals, exposure to the novel environment induced Arc mRNA levels in most pyramidal neurons in CA1, in many pyramidal neurons in CA3, and in a small number of dentate granule cells. This characteristic pattern of induction was absent at early time points after unilateral EC lesions (4 and 8 d) but recovered progressively at later time points. The recovery of Arc expression occurred with approximately the same time course as the reinnervation of the dentate gyrus as a result of postlesion sprouting. These results document a novel approach for quantitatively assessing activity-regulated gene expression in polysynaptic circuits after trauma.

SNAP-25 reduction in the hippocampus of patients with schizophrenia

In this study, the authors sought to replicate the findings of reduced synaptosomal associated protein 25 kDa (SNAP-25) immunoreactivity in the hippocampus of patients with schizophrenia. The authors also measured N-methyl-D-aspartate (NMDA) receptor 1 (NR1) receptor subunit to determine if glutamatergic synapses were involved with the loss of SNAP-25. We found 49% less SNAP-25 immunointensity in the schizophrenic group (n=7) compared to the control (n=8) or bipolar groups (n=4) (P=.004). There was no change in NMDA NR1 levels in the three groups. The authors confirm the previous report of less SNAP-25 immunoreactivity in the hippocampus using a different cohort of patients with schizophrenia. It also appears that NMDA NR1 was unchanged, indicating that the overall level of NMDA glutamatergic synapses in hippocampus is normal. These data add to evidence suggesting that in schizophrenia the molecular pathology of the hippocampus involves presynaptic components.

Differences in lesion-induced hippocampal plasticity between mice and rats

We studied the differences between mice and rats in lesion-induced sprouting in the hippocampus. The entorhinal cortex was unilaterally lesioned with ibotenic acid in adult, female mice and rats. Four weeks later the subsequent axonal sprouting in the dentate gyrus was analysed, by measuring the density of the synaptophysin immunohistochemical and acetylcholinesterase histochemical staining in the termination area of the entorhinal cortex axons. The data demonstrate that both mice and rats display a significantly increased density of staining for synaptophysin and acetylcholinesterase in the molecular layer of the dentate gyrus, indicative of axonal sprouting. Both species also show an upregulation in the density of staining for acetylcholinesterase in the molecular layer of the dentate gyrus. Further, rats, but not mice, show a significant upregulation of synaptophysin staining in stratum lacunosum moleculare of CA1 following the lesions. However, whereas rats show significant shrinkage of the molecular layer of the dentate gyrus, mice do not show any shrinkage of that layer following entorhinal cortex lesions. Taken together, these data indicate that whereas the process of reinnervation in the hippocampus is similar between the mouse and the rat, the hippocampal response to denervation shows clear differences between these two species.

Up-regulation of cystatin C expression in the murine hippocampus following perforant path transections

Cystatins are endogenous cysteine protease inhibitors that modulate the turnover of intracellular and extracellular proteins. These inhibitors are strongly implicated in a variety of pathological processes such as tumor metastasis and many degenerating CNS disorders. Here we report the expression of cystatin C, a major cysteine protease inhibitor of mammalian animals, in the murine hippocampus at 3, 7, 15 and 30 days following perforant path transections. Northern blot analysis showed that cystatin C transcripts were up-regulated in a transient manner with a significant increase at 7 and 15 days post-lesion (219% and 185% of control, respectively) in the rat hippocampus after entorhinal deafferentation. In situ hybridization and immunohistochemistry confirmed the time-dependent up-regulation of both cystatin C mRNA and protein expressions in a mouse model which initiated at 3 days post-lesion, reached maximal levels 7-15 days post-lesion, and remained slightly elevated by day 30 post-lesion. The modulation of cystatin C expression was observed to occur specifically in the entorhinally denervated zones: the stratum lacunosum-moleculare of the hippocampus and the outer molecular layer of the dentate gyrus. Double labeling by either a combination of in situ hybridization for cystatin C with immunohistochemistry for glial fibrillary acidic protein or double immunofluorescence staining for both proteins in mouse hippocampus at 7 and 15 days post-lesion revealed that most cystatin C-expressing cells are astrocytes. From these results we suggest that the spatiotemporal up-regulation of cystatin C in the hippocampus is induced by entorhinal deafferentation and that cystatin C may be involved in the astroglia-mediated neural plasticity events in the hippocampus following perforant path transections.

Low levels of estrogen significantly diminish axonal sprouting after entorhinal cortex lesions in the mouse

This study tested the hypothesis that estrogen enhances axonal sprouting in the hippocampal formation in the female mouse. The entorhinal cortex was unilaterally lesioned with ibotenic acid in control mice and in ovariectomized mice that were treated with a high dose of, a moderate dose of, or zero estrogen supplementation pellets. Four weeks later the density of staining for synaptophysin immunoreactivity and acetylcholinesterase (AChE) histochemistry was measured in the molecular layer of the dentate gyrus. In control mice, lesions of the lateral part of the entorhinal cortex increased synaptophysin and acetylcholinesterase staining (i.e., indicative of axonal sprouting) in the outer one-third of the molecular layer of the dentate gyrus. Mice receiving high and moderate estrogen supplementation displayed the same sprouting response; however, in ovariectomized mice the sprouting response was significantly reduced (to nearly nothing). Thus, in ovariectomized compared with control mice the lesion-induced sprouting response is severely blunted, and this effect is reversed by estrogen supplementation. Together, these findings suggest that estrogen plays a prominent role in promoting neuronal plasticity and remodeling in the dentate gyrus.

Transgenic mice expressing the human presenilin 1 gene demonstrate enhanced hippocampal reorganization following entorhinal cortex lesions

We have examined the effects of the presence of the mutated human presenilin 1 gene (M146L; hps1*) on lesion-induced sprouting in the hippocampus of the mouse (C57/CBA). The entorhinal cortex was unilaterally lesioned with ibotenic acid in adult, male mice. Four weeks later the subsequent axonal sprouting in the dentate gyrus was analysed, by measuring the density of the synaptophysin immunocytochemical staining in the termination area of the entorhinal cortex axons. The data demonstrate that mice expressing either the human presenilin 1 gene (hps1) or the hps1* gene display a significantly increased density of immunocytochemical staining for synaptophysin, indicative of axonal sprouting, compared to the control mice. No (or a very small) sprouting response is observed in mice expressing the normal mouse ps1 gene. Taken together, these data indicate that the presence of a human ps1 gene, normal or with an Alzheimer's disease mutation, leads to enhanced plasticity in the mouse brain.

Matrix metalloproteinase-9 undergoes expression and activation during dendritic remodeling in adult hippocampus

Neurons of adult brain are able to remodel their synaptic connections in response to various stimuli. Modifications of the peridendritic environment, including the extracellular matrix, are likely to play a role during synapse remodeling. Proteolytic disassembly of ECM is a complex process using the regulated actions of specific extracellular proteinases. One of best-characterized families of matrix-modifying enzymes is the matrix metalloproteinase (MMP) family. Here, we describe changes in the expression and function of two well known MMPs, MMP-9 and MMP-2, in adult rat brain before and after systemic administration of the glutamate receptor agonist kainate. Kainate application results in enhanced synaptic transmission and seizures followed by selective tissue remodeling, primarily in hippocampal dentate gyrus. MMP-9 but not MMP-2 was highly expressed by neurons in normal adult rat brain. MMP-9 protein was localized in neuronal cell bodies and dendrites. Kainate upregulated the level of MMP-9 mRNA and protein within hours after drug administration. This was followed several hours later by MMP-9 enzymatic activation. Within hippocampus, MMP-9 mRNA and activity were increased selectively in dentate gyrus, including its dendritic layer. In addition, MMP-9 mRNA levels decreased in areas undergoing neuronal cell loss. This unique spatiotemporal pattern of MMP-9 expression suggests its involvement in activity-dependent remodeling of dendritic architecture with possible effects on synaptic physiology.

Molecular and functional analysis of hyperpolarization-activated pacemaker channels in the hippocampus after entorhinal cortex lesion

Differential display of hippocampal tissue after entorhinal cortex lesion (ECL) revealed decreases in mRNA encoding the neuronal hyperpolarization-activated, cyclic nucleotide-gated channel HCN1. In situ hybridization confirmed that hippocampal transcripts of HCN1, but not HCN2/3/4, are down-regulated after ECL. Expression recovered at approximately 21 days after lesion (dal). Immunohistochemistry demonstrated a corresponding regulation of HCN1 protein expression in CA1-CA3 dendrites, hilar mossy cells and interneurons, and granule cells. Patch-clamp recordings in the early phase after lesion from mossy cells and hilar interneurons revealed an increase in the fast time constant of current activation and a profound negative shift in voltage activation of Ih. Whereas current activation recovered at 30 dal, the voltage activation remained hyperpolarized in mossy cells and hilar interneurons. Granule cells, however, were devoid of any detectable somatic Ih currents. Hence, denervation of the hippocampus decreases HCN1 and concomitantly the Ih activity in hilar neurons, and the recovery of h-current activation kinetics occurs parallel to postlesion sprouting.

Neuropathological findings in a patient with epilepsy and the Parry-Romberg syndrome

PURPOSE

The Parry-Romberg syndrome is an unusual disorder frequently associated with epilepsy. The origin of this disease, and the cause of epilepsy, are unknown. This study is the first reported case of the Parry-Romberg syndrome, with intractable temporal lobe epilepsy, in which detailed microanatomic analyses have been performed on resected brain tissue obtained after surgical intervention.

METHODS

Standard histopathologic methods and correlative light and electron microscopy, combined with immunocytochemical techniques, were used to study in detail the synaptic microorganization of the resected hippocampal formation.

RESULTS

After surgery, the patient was seizure free (follow-up period of 4 years and 7 months). The resected temporal lobe showed a variety of dramatic microanatomic alterations (small groups of ectopic cells, neuronal loss, gliosis, and activated microglial cells) in mesial structures, including the entorhinal cortex, subiculum, and dentate gyrus. At the electron-microscopic level, we found that in the dentate gyrus, the number of synapses in the cell-sparse region adjacent to the ectopic mass of neurons was almost twice that found in the molecular and polymorph cell layers, indicating the intrusion of neuritic processes and synapse formation. In addition, the symmetrical axosomatic synapses characteristically found on granule cells, which are likely derived from gamma-aminobutyric acid (GABA)ergic inhibitory basket cells, were not observed.

CONCLUSION

The complete seizure relief after surgery suggests that the pacemaker region(s) of seizure activity were within the resected tissue. However, we do not know which of the multiple neuropathologic findings reported here were the primary cause of seizure activity. Nevertheless, the changes found in the dentate gyrus circuitry appear to be among the most important alterations that would lead to epilepsy.

Localization of two major GABA(A) receptor subunits in the dentate gyrus of the rat and cell type-specific up-regulation following entorhinal cortex lesion

GABA(A) receptor subunits show a specific regional distribution in the CNS during development and in the adult animal. In the hippocampal formation, individual subsets of GABAergic interneurons are highly immunoreactive for the alpha1-subunit, whereas granule and pyramidal cells show a strong expression of the alpha2-subunit. Using confocal microscopy and digital image analysis, we demonstrate that in the dentate gyrus the alpha1-subunit immunolabeling appears in differently sized clusters. The large clusters, which are confined to dendrites of interneurons, show no alpha2 labeling, whereas the smaller ones coincide with alpha2-subunit-positive clusters. In the molecular layer, the clusters of both alpha-subunits co-localize with the anchoring protein gephyrin. In the granule cell layer and hilus, we found alpha1- and alpha2-subunit-positive clusters which were devoid of gephyrin labeling. Lesions of the medial entorhinal cortex led to the deafferentation of dendrites in the middle molecular layer of the dentate gyrus. This resulted in a significantly increased concentration of alpha2-subunit-positive clusters. We also observed an increase of alpha1-subunit immunolabeling in the deafferented area. We found no change in the co-localization between alpha1 and alpha2, and no significant change in the number of large alpha1-positive clusters along individual dendritic segments of interneurons. In a previous study, we demonstrated that calbindin-immunoreactive dendrites of granule cells revealed a significant increase in gephyrin immunoreactivity following lesion, whereas parvalbumin-positive dendrites showed no such alterations. The predominant localization of small gephyrin clusters in dendrites of granule cells, which was also described in this study, leads to the conclusion that the increase of the alpha2-subunit-positive clusters, demonstrated in the present study, indicates that, following entorhinal cortex lesion, new GABAergic synapses may be formed and that they contact predominantly granule cell dendrites.

Perforant path lesion induces up-regulation of stathmin messenger RNA, but not SCG10 messenger RNA, in the adult rat hippocampus

In this study, we performed in situ hybridization analysis of the expression pattern of two growth-associated proteins, stathmin and SCG10, in the hippocampus after unilateral lesion of the perforant pathway, the main excitatory input from the entorhinal cortex to the hippocampus. Stathmin is one of the major neural-enriched cytosolic phosphoproteins and a potential target of cyclic-AMP-dependent kinases [Jin L. W. et al. (1996) Neurobiol. Aging 17, 331-341; Leighton I. A. et al. (1993) Molec. Cell Biochem. 127/128, 151-156]. Three days after the lesion, stathmin messenger RNA was up-regulated ipsilaterally in the hilus, in the granule cell layer of the dentate gyrus and in the pyramidal cell layer of the CA1 region. Simultaneously, the hilar region of the contralateral dentate gyrus showed an increased stathmin messenger RNA expression. This altered expression pattern was observed until 15 days after lesion. Stathmin messenger RNA expression returned to a normal level until 21 days after lesion in all regions analysed. SCG10, a membrane-bound neuronal growth-associated protein belonging to the SCG10/stathmin gene family, did not show any alteration of messenger RNA expression after perforant path lesion. The temporal changes of stathmin messenger RNA expression in the ipsilateral hippocampus correspond well to the process of reactive synaptogenesis. The enhanced messenger RNA expression in the hilar region of the contralateral dentate gyrus might suggest a role in neurite elongation, since this region is the origin of commissural fibres involved in the sprouting response in the deafferented hippocampus. The present study provides evidence that the induction of specific growth-associated proteins is differentially regulated in the hippocampus.

Deafferentation of the hippocampus results in the induction of AT8 positive 'granules' in the rat

Hyperphosphorylated tau is a pathological hallmark of Alzheimer's disease, but the mechanisms that lead to its formation are poorly understood. To investigate what effect deafferentation of the hippocampus has on the phosphorylation state of tau, we lesioned the entorhinal cortex in rats and looked for hyperphosphorylated tau in the hippocampus at various days post lesioning. After 7 and 21 days, small AT8-positive 'granules' appeared in the molecular layer of the dentate gyrus on the lesioned side. No such staining was seen in the animals injected with saline. This study shows that deafferentation leads to induction of hyperphosphorylated tau. The AT8 positive 'granules' seen resemble the argyrophilic grains that characterize Argyrophilic Grain disease suggesting that lesioning the perforant pathway may serve as a useful model for inducing argyrophilic grains in vivo.

Regulation of dendritic spine stability

Dendritic spines undergo several types of transformations, ranging from growth to collapse, and from elongation to shortening, and they experience dynamic morphological activity on a rapid time scale. Changes in spine number and morphology occur under pathological conditions like excitotoxicity, but also during normal central nervous system development, during hormonal fluctuations, and in response to neural activity under physiological circumstances. We briefly review evidence for various types of alterations in spines, and discuss the possible molecular basis for changes in spine stability. Filamentous actin appears to be the most important cytoskeletal component of spines, and a growing list of actin-associated and actin-regulatory proteins has been reported to reside within spines. We conclude that spines contain two distinct pools of actin filaments (one stable, the other unstable) that provide the spine with both a stable core structure and a dynamic, complex shape. Finally, we review the current state of knowledge of actin filament regulation, based on studies in nonneuronal cells.

IFNgamma enhances microglial reactions to hippocampal axonal degeneration

Glial reactivity is implicated in CNS repair and regenerative responses. Microglia, the cells responding earliest to axonal injury, produce tumor necrosis factor-alpha (TNFalpha), a cytokine with both cytopathic and neuroprotective effects. We have studied activation of hippocampal microglia to produce TNFalpha in response to transection of perforant path axons in SJL/J mice. TNFalpha mRNA was produced in a transient manner, peaking at 2 d and falling again by 5 d after lesioning. This was unlike other markers of glial reactivity, such as Mac-1 upregulation, which were sustained over longer time periods. Message for the immune cytokine interferon-gamma (IFNgamma) was undetectable, and glial reactivity to axonal lesions occurred as normal in IFNgamma-deficient mice. Microglial responses to lesion-induced neuronal injury were markedly enhanced in myelin basic protein promoter-driven transgenic mice, in which IFNgamma was endogenously produced in hippocampus. The kinetics of TNFalpha downregulation 5 d after lesion was not affected by transgenic IFNgamma, indicating that IFNgamma acts as an amplifier and not an inducer of response. These results are discussed in the context of a regenerative role for TNFalpha in the CNS, which is innately regulated and potentiated by IFNgamma.

Axonal sprouting regulates myelin basic protein gene expression in denervated mouse hippocampus

The regulation of oligodendrocyte gene expression and myelination in vivo in the normal and injured adult CNS is still poorly understood. We have analyzed the effects of axotomy-induced axonal sprouting and microglial activation, on oligodendrocyte myelin basic protein (MBP) gene expression from 2 to 35 days after transection of the entorhino-hippocampal perforant path axonal projection. In situ hybridization analysis showed that anterograde axonal and terminal degeneration lead to upregulated oligodendrocyte MBP mRNA expression starting between day 2 and day 4, in (1) the deep part of stratum radiatum of CA3 and the dentate hilus, which display axonal sprouting but no degenerative changes or microglial activation, and (2) the outer part of the molecular layer of the fascia dentata, and in stratum moleculare of CA3 and stratum lacunosum-moleculare of CA1, areas that display dense anterograde axonal and terminal degeneration, myelin degenerative changes, microglial activation and axotomi-induced axonal sprouting. Oligodendrocyte MBP mRNA expression reached maximum in both these areas at day 7. MBP gene transcription remained constant in stratum radiatum, stratum pyramidale and stratum oriens of CA1, areas that were unaffected by perforant path transection. These results provide strong evidence that oligodendrocyte MBP gene expression can be regulated by axonal sprouting independently of microglial activation in the injured adult CNS.

Outgrowth-promoting molecules in the adult hippocampus after perforant path lesion

Lesion-induced neuronal plasticity in the adult central nervous system of higher vertebrates appears to be controlled by region- and layer-specific molecules. In this study we demonstrate that membrane-bound hippocampal outgrowth-promoting molecules, as present during the development of the entorhino-hippocampal system and absent or masked in the adult hippocampus, appear 10 days after transection of the perforant pathway. We used an outgrowth preference assay to analyse the outgrowth preference of axons from postnatal entorhinal explants on alternating membrane lanes obtained from hippocampus deafferented from its entorhinal input taken 4, 10, 20, 30 and 80 days post-lesion and from adult control hippocampus. Neurites from the entorhinal cortex preferred to extend axons on hippocampal membranes disconnected from their entorhinal input for 10 days in comparison with membranes obtained from unlesioned adult animals. Membranes obtained from hippocampi disconnected from their entorhinal input for 10 days were equally as attractive for growing entorhinal cortex (EC) axons as membranes from early postnatal hippocampi. Further analysis of membrane properties in an outgrowth length assay showed that entorhinal axons extended significantly longer on stripes of lesioned hippocampal membranes in comparison with unlesioned hippocampal membranes. This effect was most prominent 10 days after lesion, a time point at which axonal sprouting and reactive synaptogenesis are at their peak. Phospholipase treatment of membranes obtained from unlesioned hippocampi of adult animals strongly promoted the outgrowth length of entorhinal axons on these membranes but did not affect their outgrowth preference for deafferented hippocampal membranes. Our results indicate that membrane-bound outgrowth-promoting molecules are reactivated in the adult hippocampus following transection of the perforant pathway, and that neonatal entorhinal axons are able to respond to these molecules. These findings support the hypothesis of a temporal accessibility of membrane-bound factors governing the layer-specific sprouting of remaining axons following perforant path lesion in vivo.

Synaptic reorganization in the hippocampal formation of alcohol-fed rats may compensate for functional deficits related to neuronal loss

We have examined the behavioral and neuroanatomical effects of long-term alcohol intake in rats ingesting a 20% solution of ethanol for 30 weeks. Previous studies have shown that this treatment provokes neuronal degeneration in the hippocampal formation, which occurs in parallel with remodeling processes. Spatial reference and working memory of alcohol-fed rats were evaluated during last 4 weeks of treatment by comparison of their performance with age-matched controls on the Morris water maze. Alcohol consumption did not affect the performance of rats in the reference memory task as indicated by the measures derived from the acquisition trials and from the probe-trial, which were highly similar for alcohol-fed and control animals. Also, performance in the working memory task was not significantly altered in alcohol-treated animals. No treatment-related changes in swim speed or impairments of sensorimotor abilities, tested in the visible platform task, were detected. Stereological methods were applied to evaluate the damage inflicted by alcohol intake in the structure of the hippocampal formation. In the alcohol-treated animals, there was a noticeable cell loss in the granular layer of the dentate gyrus (10%), and in CA3 (18%) and CA1 (19%) hippocampal subdivisions. In spite of the neuronal loss, the total number of synapses between mossy fibers and CA3 pyramids was unaffected by alcohol treatment suggesting that new synaptic contacts were formed between the surviving neurons. We show that, regardless the marked hippocampal cell loss in rats exposed to chronic alcohol intake, the reorganization that takes place at the synaptic level may alleviate the expected functional deficits.

Entorhinal cortex lesion in adult rats induces the expression of the neuronal chondroitin sulfate proteoglycan neurocan in reactive astrocytes

The chondroitin sulfate proteoglycan neurocan is a major component of brain extracellular matrix during development. Neurocan is primarily synthesized by neurons and has the ability to interact with cell adhesion molecules involved in the regulation of cell migration and axonal growth. Within the first weeks postnatally, neurocan expression is strongly downregulated. To test whether neurocan is reexpressed in areas of axonal growth (sprouting) after brain injury, the time course of neurocan expression was analyzed in the denervated fascia dentata of the rat after entorhinal cortex lesion (12 hr; 1, 2, 4, and 10 d; 2 and 4 weeks; and 6 months after lesion). In the denervated zone, immunohistochemistry revealed neurocan-positive astrocytes by 2 d after lesion and a diffuse labeling of the extracellular matrix at all later time points. Electron microscopy confirmed the deposition of neurocan in the extracellular matrix compartment. In situ hybridization demonstrated a strong upregulation of neurocan mRNA within the denervated outer molecular layer 1 and 4 d after lesion. The combination of in situ hybridization with immunohistochemistry for glial fibrillary acidic protein demonstrated that the neurocan mRNA-expressing cells are astrocytes. These data demonstrate that neurocan is reexpressed in the injured brain. In contrast to the situation during development, astrocytes, but not neurons, express neurocan and enrich the extracellular matrix with this molecule. Similar to the situation during development, neurocan is expressed in an area of active axon growth, and it is suggested that neurocan acts to maintain the boundaries of the denervated fascia dentata after entorhinal cortex lesion.

Neocortical and hippocampal glucose hypometabolism following neurotoxic lesions of the entorhinal and perirhinal cortices in the non-human primate as shown by PET. Implications for Alzheimer's disease

Temporoparietal glucose hypometabolism, neuronal loss in the basal forebrain cholinergic structures and preferential accumulation of neurofibrillary tangles in the rhinal cortex (i.e. in the entorhinal and perirhinal cortices) are three early characteristics of Alzheimer's disease. Based on studies of the effects of neurotoxic lesions in baboons, we previously concluded that damage to the cholinergic structures plays, at best, a marginal role in the association neocortex hypometabolism of Alzheimer's disease. In the present study, we have assessed the remote metabolic effects of bilateral neurotoxic lesions of both entorhinal and perirhinal cortices. Using coronal PET coregistered with MRI, the cerebral metabolic rate for glucose (CMR(glc)) was measured before surgery and sequentially for 2-3 months afterward (around days 30, 45 and 80). Compared with sham-operated baboons, the lesioned animals showed a significant and long-lasting CMR(glc) decline in a small set of brain regions, especially in the inferior parietal, posterior temporal, posterior cingulate and associative occipital cortices, as well as in the posterior hippocampal region, all of which also exhibit glucose hypometabolism in Alzheimer's disease. Remarkably, the degree of CMR(glc) decline in four of these regions significantly correlated with the severity of histologically determined damage in the rhinal cortex, strongly supporting the specificity of the observed metabolic effects. There were also differences between the metabolic pattern observed in the lesioned animals and that classically reported in Alzheimer's disease; for instance, the hypometabolism we found in the stratum has not been reported in early Alzheimer's disease, although this structure can be affected in late stages of the disease and has direct anatomical connections with the rhinal cortex. Nevertheless, this study shows for the first time that the temporoparietal and hippocampal hypometabolism found in Alzheimer's disease may partly result from neuroanatomical disconnection with the rhinal cortex. This, in turn, further strengthens the hypothesis that neuronal damage and dysfunction in the rhinal cortices play a major role in the expression of Alzheimer's disease.

Cerebral glucose metabolism in unilateral entorhinal cortex-lesioned rats: an animal PET study

To evaluate the effect of entorhinal cortical lesion on cerebral cortical function, we studied cerebral glucose utilization (CMRGlc) using a high resolution PET scanner after quinolinic acid lesion of the unilateral entorhinal cortex in rats. [18F]Fluorodeoxyglucose PET was performed at 4 days and 4 weeks after surgery, and CMRGlc in the bilateral frontal, parietal and temporal regions were analyzed. At 4 days, the entorhinal lesion induced a 12-15% decrease in CMRGlc of frontal, parietal and temporal regions ipsilateral to the lesion. The hypometabolism continued at 4 weeks in the temporal region. These findings suggest that entorhinal lesion induces cerebral cortical hypometabolism, which implies a pathogenetic role of entorhinal area on the cortical hypometabolism in Alzheimer's disease.

Dexamethasone selectively suppresses microglial trophic responses to hippocampal deafferentation

Hippocampal deafferentation increases the expression of insulin-like growth factor-1 by microglia, and of ciliary neurotrophic factor and basic fibroblast growth factor by astroglia in fields and periods of reactive axonal growth. Glucocorticoids attenuate lesion-induced hippocampal sprouting, possibly by reducing trophic signals that stimulate growth. With an interest in this hypothesis, the present studies evaluated the influence of systemic treatment with the synthetic glucocorticoid dexamethasone on entorhinal lesion-induced increases in neurotrophic factor expression in young adult rat hippocampus. Daily dexamethasone injections almost completely blocked increases in insulin-like growth factor-1 messenger RNA content, but did not perturb increases in ciliary neurotrophic factor or basic fibroblast growth factor messenger RNA content, in the deafferented dentate gyrus molecular layer. To determine if the suppression of insulin-like growth factor-1 expression was secondary to a general inhibition of microglial responses, and to identify the time period of glucocorticoid sensitivity, additional rats were prepared to evaluate the effects of semi-chronic (i.e. daily) and single dexamethasone injections on microglial proliferation, ED-1 immunoreactivity (a marker of microglial reactivity) and insulin-like growth factor-1 messenger RNA expression. Semi-chronic dexamethasone treatment attenuated all three measures of deafferentation-induced microglial reactivity. However, a single dexamethasone injection given two (but not one or three) days postlesion inhibited deafferentation-induced increases in insulin-like growth factor-1 messenger RNA content, without having significant effects on other measures. These results demonstrate that dexamethasone treatment preferentially suppresses microglial, as opposed to astroglial, trophic responses to deafferentation, and suggest that glucocorticoids attenuate reactive axonal sprouting by inhibiting the microglial production of insulin-like growth factor-1.

Quantitative evidence for increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions in the adult rat

The projection from the entorhinal cortex to the dentate gyrus and hippocampus is severely affected in Alzheimer's disease and there is a depletion of cholinergic terminals but an upregulation of the neuropeptide galanin, which inhibits the release of acetylcholine. Evidence for changes to galanin-immunoreactive terminals in the hippocampal formation was therefore examined after unilateral entorhinal cortex lesions in the adult rat. An increase in the density of galanin-immunoreactive terminals on the lesioned side was evident in the stratum lacunosum moleculare of the hippocampus and the outer molecular layer of the dentate gyrus at 17 days post-lesion, and it increased gradually until the last time point examined, at 40 days post-lesion. Thus we demonstrate that there is an increase in galanin-immunoreactive terminals in the hippocampal formation following entorhinal cortex lesions.

S100B, a neurotropic protein that modulates neuronal protein phosphorylation, is upregulated during lesion-induced collateral sprouting and reactive synaptogenesis

Using light and electron microscopic immunocytochemistry, we examined the expression of the Ca2+-binding protein S100B in the dentate gyrus of adult rats during lesion-induced sprouting and reactive synaptogenesis. Nine days following unilateral lesioning of the entorhinal cortex, S100B was upregulated in cells primarily in the outer part of the molecular layer of the ipsilateral dentate gyrus. When examined with electron microscopy, numerous astrocytes and synapses containing S100B were identified. These data show that during lesion-induced sprouting and reactive synaptogenesis, S100B is upregulated in astrocytes and can be found in pre- and post-synaptic compartments where it might influence neuronal protein phosphorylation.

Morphological features of the entorhinal-hippocampal connection

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Density of choline acetyltransferase-immunoreactive terminals in the rat dentate gyrus after entorhinal cortex lesions: a quantitative light microscope study

Lesion of the entorhinal cortex in the adult rat is a model for Alzheimer's disease and produces a marked increase in acetylcholinesterase (AChE) activity in the outer molecular layer (OML) of the dentate gyrus. This has been attributed to the sprouting of cholinergic axons terminals in response to denervation of the OML. The aim of this study was to investigate the density changes of cholinergic terminals in the OML at the light microscope level by using choline acetyltransferase (ChAT) immunohistochemistry and quantitative analysis. The results showed that between days 10 and 33 after an entorhinal cortex lesion, there was a measurable increase in the density of ChAT-positive boutons in the OML of the ipsilateral dentate gyrus (x1.2-1.6 of contralateral). However, when shrinkage of the ipsilateral OML (x0.5-0.75 of contralateral) was taken into account, the apparent increase in ChAT terminal density was entirely accounted for by shrinkage of the OML. Thus ChAT immunohistochemistry at the light microscope level provides no positive evidence for a proliferation of cholinergic terminals in the entorhinal cortex lesion model. This is in agreement with previous biochemical assays that have shown no change of total ChAT activity in the dentate gyrus after entorhinal cortex lesions.

Free postsynaptic densities in the hippocampus of the female rat

The number of synapses in the adult, female hippocampal CA1 region fluctuates naturally across the estrous cycle in an ovarian steroid-dependent manner. This phasic variation in synapse number occurs without identifiable degenerating synapses. Ultrastructural correlates of the dynamic aspect of this synapse loss and synapse formation thus remain undescribed. During early development, one hallmark of synaptogenesis is the presence of free postsynaptic densities (PSDs). Here we report that the incidence of free PSDs in CA1 fluctuates across the rat estrous cycle. The number of free PSDs is greatest on the afternoon of proestrus and is significantly decreased on the afternoon of estrus, 24 h later. We hypothesize that these free PSDs reflect synapse turnover in the adult CA1 region.

The effect of age and testosterone on the expression of glial fibrillary acidic protein in the rat cerebellum

Testosterone reversed the age-related increase in glial fibrillary acidic protein (GFAP) in the male rat cerebellum, a brain region not generally associated with gonadal steroid hormone sensitivity. This supports the hypothesis that a decrease in circulating testosterone contributes to age-related increase in GFAP. These data also suggest that reductions in circulating gonadal steroids during aging could render the brain more susceptible to neurodegeneration and that hormone replacement therapy might have value in neurodegenerative disease intervention.

Up-regulation of astrocyte-derived tenascin-C correlates with neurite outgrowth in the rat dentate gyrus after unilateral entorhinal cortex lesion

The extracellular matrix protein tenascin-C has been implicated in the regulation of axonal growth. Using unilateral entorhinal cortex lesions, which induce a massive sprouting response in the denervated outer molecular layer of the rat fascia dentata, the role of tenascin-C for axonal growth was investigated in vivo. Monoclonal antibodies against the neurite outgrowth and anti-adhesive domains of the molecule were employed. Immunostaining was increased throughout the denervated outer molecular layer by day 2, reached a maximum around day 10, and was back to control levels by four weeks post lesion. Growth cone deflecting as well as neurite outgrowth promoting isoforms of tenascin-C were up-regulated after the lesion. Using electron microscopy, single intensely tenascin-C immunoreactive cells were identified as reactive astrocytes that phagocytose degenerated terminals. In situ hybridization histochemistry for tenascin-C messenger RNA revealed numerous cellular profiles in the denervated outer molecular layer of the ipsilateral and contralateral dentate gyrus two days post lesion. Tenascin-C messenger RNA-positive cells in the outer molecular layer were identified as astrocytes using double-labelling for tenascin-C messenger RNA and glial fibrillary acidic protein immunohistochemistry. Thus, a tenascin-C-rich substrate is present in the outer molecular layer during the time of sprouting and a sharp boundary is formed against the inner molecular layer. This pattern may contribute to the layer-specific sprouting response of surviving afferents after entorhinal lesion. Neurite outgrowth may be promoted within the denervated zone, whereas axons trying to grow into the denervated outer molecular layer, for example from the inner molecular layer, would be deflected by a tenascin-C-rich barrier.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Basal expression, subcellular distribution, and up-regulation of the proto-oncogene c-JUN in the rat dentate gyrus after unilateral entorhinal cortex lesion

The expression of the transcription factor c-JUN was investigated in the rat fascia dentata under normal conditions and after entorhinal cortex lesion. As shown by immunocytochemistry and in situ hybridization histochemistry c-JUN and its messenger RNA are present in the principal cell layers of the dentate gyrus and Ammon's horn (except hippocampal region CA2). Pre-embedding immunogold electron microscopy revealed an almost exclusive nuclear localization of c-JUN, where it is associated with chromatin. In addition, double immunolabelling for c-JUN and parvalbumin demonstrated that c-JUN immunoreactivity is primarily found in principal neurons since GABAergic parvalbumin-positive interneurons did not express c-JUN. After unilateral electrolytic lesion of the entorhinal cortex c-JUN was strongly up-regulated in the ipsilateral dentate gyrus within 2 h postlesion. This up-regulation was also present in the contralateral fascia dentata 12 h after entorhinal cortex lesion and returned to control levels on both sides 24 h postlesion. The cellular distribution of c-JUN did not change after entorhinal cortex lesion: parvalbumin-positive interneurons never contained c-JUN. These results point to a specific role of c-JUN in the granule cells of the fascia dentata in the normal animal and in rats with entorhinal cortex lesions. The selective induction of c-JUN after entorhinal lesion could be one of the first molecular steps that regulate transneuronal changes within granule cells after their denervation. A different mechanism has to be assumed for GABAergic interneurons known to receive an entorhinal innervation as well.

Staurosporine but not chelerythrine inhibits regeneration in hippocampal organotypic cultures

A mechanical lesion in hippocampal organotypic cultures is followed by a recovery process involving scar formation, sprouting of fibres and formation of new functional synapses. Here we tested the effect of staurosporine and chelerythrine, two protein kinase C (PKC) inhibitors, on this lesion-induced neurite outgrowth of Shaffer collaterals. At a concentration of 1 microM, staurosporine delayed functional recovery assessed by measuring synaptic field potentials across the lesion, without altering synaptic transmission on nonlesioned cultures. Immunostaining carried out by using antibodies directed against neurofilament proteins showed that there was a marked reduction in the number of regenerating fibres crossing the lesion. In contrast to this, chelerythrine (50 microM) did not prevent functional recovery, although it affected synaptic transmission and plasticity at this concentration. We conclude that the inhibition of sprouting produced by staurosporine is independent of its blockade of PKC-mediated phosphorylation mechanisms.

Cholinergic sprouting in the rat fascia dentata after entorhinal lesion is not linked to early changes in neurotrophin messenger RNA expression

After unilateral entorhinal cortex lesion cholinergic septohippocampal fibres sprout in the denervated fascia dentata. This process is dependent on neurotrophin changes following the lesion. Thus, there is an up-regulation of nerve growth factor and brain-derived neurotrophic factor messenger RNA expression in the denervated granule cells which is detectable 4 h postlesion and returns to control levels by 24 h. Here, using a competitive polymerase chain reaction and in situ hybridization, a transient neurotropin messenger RNA increase could be demonstrated bilaterally following unilateral electrolytic entorhinal cortex lesion. Treatment of the animals with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked this messenger RNA increase, suggesting an involvement of this receptor type in the neurotrophin changes. However, in spite of this blockade, the typical cholinergic sprouting response as visualized with acetylcholinesterase histochemistry was present in animals four weeks after entorhinal cortex lesion. These data suggest that brief initial changes in neurotrophin messenger RNA expression in dentate granule cells are not responsible for the induction of the cholinergic sprouting. Changes in neurotrophin messenger RNA expression occurring immediately postlesion may be linked to glutamate release from entorhinal terminals resulting from the electrolytic lesion of the projection cells in the entorhinal cortex. We hypothesize that later changes in neurotrophin expression, for example in glial cells, are more likely to be related to the cholinergic sprouting process.

Differential spine loss and regrowth of striatal neurons following multiple forms of deafferentation: a Golgi study

Golgi-Cox method and morphometric analyses were used to study the plasticity of striatal medium spiny I neurons in 6-month-old C57BL/6N mice after unilateral or bilateral lesion of the cerebral cortex or combined lesions of the ipsilateral cerebral cortex and intralaminar thalamus. In adult mouse, unilateral lesions of the cerebral cortex did not result in a net gain or loss of linear dendritic length in a randomly selected population of striatal medium spiny I neurons. In addition, there was a well-defined time course of striatal spine loss and replacement occurring after a unilateral cortical lesion. By day 3 postlesion the average 20-microm dendritic segment had lost 30% of the unlesioned control spine value, reached its nadir, lost 45.5%, at 10 days postlesion, and recovered to 80% of unlesioned control levels by 20 days postlesion. The recovery of spines was blocked by a secondary lesion on the contralateral cortex but not on the ipsilateral intralaminar thalamus. These data suggest that striatal medium spiny I neurons of adult mice have a remarkable capacity for plasticity and reactive synaptogenesis following a decortication. The recovery of spine density is primarily induced by axonal sprouting of survival homologous afferent fibers from the contralateral cortex.

Laminin-alpha2 chain-like antigens in CNS dendritic spines

The laminin-alpha2 chain is a component of brain capillary basement membranes and appears also to be present in neurons of rat, rabbit, pig and non-human primate brain as evidenced by immunohistochemistry. In the present study, we have further characterized this very distinct neuronal laminin-alpha2 chain-like immunoreactivity in the hippocampus of various species. Immunoelectron microscopy with poly- and monoclonal antibodies to the laminin-alpha2 chain G-domain localized laminin-alpha2 chain immunoreactivity in adult rat and rabbit hippocampus to dendritic processes, primarily to dendritic spines. In the developing rat hippocampus, spine-associated laminin-alpha2 chain-like immunoreactivity first appeared at a time corresponding to that of active synaptogenesis. After an entorhinal cortex lesion in adult rats, the time course of denervation-induced loss and reactive reappearance of spines in the molecular layer of the dentate gyrus was correlated closely to the loss and reappearance of laminin-alpha2 chain immunoreactivity. Immunoblot analysis of normal adult rat, rabbit and pig brain revealed a protein similar in size to the reported 80-kDa laminin-alpha2 chain fragment of human placenta as well as 140/160-kDa proteins. These results suggest the presence of proteins with antigenic homology to the laminin-alpha2 chain and/or laminin-alpha2 isoforms in dendrites and dendritic spines in selected areas of the brain, predominately in the hippocampus and other limbic structures. Given the adhesion and neurite promoting functions of laminins, it is possible that neuronal laminin-alpha2 chain-like proteins play a role in synaptic function and plasticity in the CNS.

Injury-induced physiological events that may modulate gene expression in neurons and glia

Damage to the brain triggers a host of reactive responses in neurons and glia which are seen at sites of focal injury as well as at sites that are at a distance from the injury. Although many of these responses have been studied extensively, the signals that initiate the different responses have not been fully characterized, and it is still not understood how focal injury affects neurons and glia in distant sites. The present review summarizes recent findings that suggest that physiological events that occur at the time of the injury or during the early postlesion period can play an important and variable role in modulating neuronal and glial responses to injury. We focus on the events that occur in the hippocampal formation following unilateral lesions of the entorhinal cortex - a model system that has been used extensively for studies of cellular responses following focal brain injury. This lesion destroys the cells of origin of a massive excitatory projection to the dentate gyrus and hippocampus proper. Over time, the denervated neurons in the hippocampal formation are almost completely reinnervated as a result of local sprouting of systems that survive the lesion. Thus, this model system has been useful for studying cellular responses to both denervation and reinnervation. We summarize the information that this injury triggers physiological events that can strongly modulate gene expression in neurons and glia, including episodes of spreading depression that occur at the time of the injury, seizures that occur during the early postlesion period, the loss of afferent drive which leads to decreases in postsynaptic activity, and the restoration of activity that occurs in conjunction with reinnervation. We describe recent studies which suggest that some of these physiological events occur to a variable extent in different animals, especially the episodes of spreading depression and the recurrent seizures. Thus, the spatial pattern and temporal dynamics of altered gene expression following this "model" experimental injury may vary from animal to animal. The fact that physiological events strongly modulate the reactive changes in gene expression that occur following injury has important implications for understanding the sequelae of injury, and offers new opportunities for experimental and therapeutic interventions that may improve cellular repair, regeneration, and recovery of function.

Astrocytes and microglial cells incorporate degenerating fibers following entorhinal lesion: a light, confocal, and electron microscopical study using a phagocytosis-dependent labeling technique

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main termination zone in the outer molecular layers of the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and microglial cells alter their phenotype, suggesting participation in anterograde degeneration. This study analyzes the involvement of these lesion-induced activated glial cells in the process of phagocytosis of degenerated axonal debris. We established a phagocytosis-dependent labeling technique that allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells. Stereotaxic application of small crystals of the biotin- and rhodamine-conjugated dextran amine Mini Ruby (MR) into the entorhinal cortex led to strong and stable axonal staining of perforant path axons. Following entorhinal lesion, labeled terminals and fibers condensed and formed small granules. Incorporation of these rhodamine-fluorescent granules resulted in a phagocytosis-dependent cell labeling. During the first 3 days, we were able to identify these cells as microglia by using double-fluorescence and confocal microscopy. The first unequivocally double-labeled astrocytes were found 6 days post lesion (dpl). Whereas in all stages a subpopulation of microglial cells remained devoid of MR-labeled granules, all astrocytes in the middle molecular layer were double-labeled after long survival times (20 dpl). On the ultrastructural level, labeled granules appeared to be perforant path axons containing the tracer. Both terminals and myelinated fibers could be seen inside the cytoplasm of microglial cells and astrocytes. Thus, anterograde degeneration is a sufficient stimulus to induce axon incorporation by both astrocytes and a subpopulation of microglial cells.

Evidence for recovery of spatial learning following entorhinal cortex lesions in mice

The influence of entorhinal cortex lesions on behaviour and concommitant changes in synaptophysin immunoreactivity (IR) in the denervated dentate gyrus was assessed. Male, C57/B6 mice received either bilateral (BI), unilateral (UNI), or no lesion (SHAM) to the entorhinal cortex. At various stages post-lesion the animals were evaluated in tests to examine neurological and cognitive (spatial and cued learning, Morris water maze) function. UNI lesioned animals from 6-36 days post-lesion showed no neurological nor marked cued learning deficit, yet a profound spatial learning deficit. However by 70 days post-lesion, spatial learning ability was clearly evident. In contrast, BI lesioned animals showed severe spatial learning deficits throughout the test period (6-70 days), cued learning was also impaired. In parallel groups of UNI lesioned mice, 6-36 days post-lesion there was a marked reduction (-40%) in synaptophysin IR in the dentate gyrus molecular layer. However by 70 days post-lesion a clear increase in this measure was noted. Changes in the expression of the growth associated protein, GAP43, were also noted over this period. Taken together, the present results suggest some recovery of spatial learning following unilateral entorhinal cortex lesions in mice. This behavioural recovery of a hippocampally dependant task may be associated with a recovery of function related to the synaptic remodelling and elevation of synapse number in the denervated hippocampus, as evidenced by changes in synaptophysin and GAP43 IR.

Sprouting in the hippocampus is layer-specific

Partial removal of layer-specific afferents of the hippocampus is said to induce sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent in vivo and in vitro studies using sensitive anterograde tracers have failed to demonstrate sprouting across laminar boundaries. Sprouting does occur; but, it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration in addition to the degenerating afferents. These findings point to rigid laminar cues attracting certain fiber systems while repelling others in normal development and after partial deafferentation.

Genetic influences on cellular reactions to brain injury: activation of microglia in denervated neuropil in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study examines the relationship between the appearance of degenerative changes in synaptic terminals and axons and the activation of microglia in denervated neuropil regions of normal mice of the C57BL/6 strain and mutant mice (Wld(S)), in which Wallerian degeneration is substantially delayed. The time course of degenerative changes in synaptic terminals and axons was assessed using selective silver staining. Microglial cells were identified by immunostaining for Mac-1, a monoclonal antibody to the CR3 complement receptor, and by histochemical staining for nucleoside diphosphatase (NDPase). Increased argyrophilia, indicative of degenerative changes, was evident as early as 1 day postlesion in normal mice, but was not seen until 6-8 days in mice with the Wld(S) mutation. Microglial activation in normal C57BL/6 mice was evident by 24 hours postlesion, as evidenced by increased immunostaining for Mac-1, increased histochemical staining for NDPase, and morphological changes indicative of an activated phenotype (short, thick processes). Quantitative evaluation of immunostaining for Mac-1 revealed that peak activation occurred between 2 and 6 days postlesion with a return to a quiescent phenotype by 12 days. In contrast, the microglial response was significantly delayed and prolonged in mice bearing the Wld(S) mutation. Activated microglia were not seen within the deafferented area until 6 to 8 days postlesion and peak activation occurred between 12 and 20 days postlesion. These data suggest that the response of microglia in denervated neuropil zones is triggered by the same types of degenerative changes that cause increased argyrophilia as detected by selective silver staining methods.

Genetic influences on cellular reactions to CNS injury: the reactive response of astrocytes in denervated neuropil regions in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study compares the reactive changes in astrocytes in denervated neuropil regions in normal mice and in mice carrying the Wld(S) mutation which leads to delayed Wallerian degeneration. In situ hybridization and immunocytochemical techniques were used to define the time course of changes in the levels of glial fibrillary acidic protein (GFAP) and GFAP mRNA in the denervated neuropil of the hippocampus after unilateral aspiration lesions of the entorhinal cortex. In control mice, GFAP mRNA levels increased rapidly in the denervated neuropil to a peak that was about tenfold higher than control at 2-4 days, decreased between 6 and 8 days postlesion, and then increased again to a second peak at 10 days postlesion. Increases in immunostaining for GFAP were evident by 2 days, remained elevated until 12 days postlesion and then decreased slowly. In mice carrying the Wld(S) mutation, the upregulation of GFAP mRNA levels in the denervated laminae was substantially delayed. Strikingly absent was the dramatic increase in labeling at 2-4 days postlesion which was such a prominent feature of the response in control animals. Peak labeling in the denervated laminae was not seen until 10-12 days postlesion. The development of a well-defined band of intensely immunostained and hypertrophied astrocytes in the denervated zone was also delayed in the Wld(S) animals, although there were modest increases in immunostaining as early as 2 days postlesion that were seen throughout the hippocampus ipsilateral to the lesion. These results suggest that degenerative changes in axons and synaptic terminals are the principal trigger for upregulating GFAP expression in the denervated neuropil, although other signals also play a role in the early postlesion response.

Differential regulation of ciliary neurotrophic factor (CNTF) and CNTF receptor alpha expression in astrocytes and neurons of the fascia dentata after entorhinal cortex lesion

Neurotrophic factors have been implicated in reactive processes occurring in response to CNS lesions. Ciliary neurotrophic factor (CNTF), in particular, has been shown to ameliorate axotomy-induced degeneration of CNS neurons and to be upregulated at wound sites in the brain. To investigate a potential role of CNTF in lesion-induced degeneration and reorganization, we have analyzed the expression of CNTF protein and CNTF receptor alpha (CNTFR alpha) mRNA in the rat dentate gyrus after unilateral entorhinal cortex lesions (ECLs), using immunocytochemistry and nonradioactive in situ hybridization, respectively. In sham-operated as in normal animals, CNTF protein was not detectable by immunocytochemistry. Starting at 3 d after ECL, upregulation of CNTF expression was observed in the ipsilateral outer molecular layer (OML). Expression was maximal at around day 7, and at this stage immunoreactivity could be specifically localized to astrocytes in the ipsilateral OML. By day 14 postlesion, CNTF immunoreactivity had returned to control levels. CNTFR alpha mRNA was restricted to neurons of the granule cell layer in controls. Three days postlesion, prominent CNTFR alpha expression was observed in the deafferented OML. A similar but less prominent response was noticed in the contralateral OML. After 10 d, CNTFR alpha expression had returned to control levels. Double labeling for CNTFR alpha mRNA and glial fibrillary acidic protein (GFAP) showed that upregulation of CNTFR alpha occurred in reactive, GFAP-immunopositive astrocytes of the OML. A substantial reduction of CNTFR alpha expression in the deafferented granule cells was transiently observed at 7 and 10 d postlesion. Our results suggest a paracrine or autocrine function of CNTF in the regulation of astrocytic and neuronal responses after brain injury.

Dimensions and density of dendritic spines from rat dentate granule cells based on reconstructions from serial electron micrographs

In the hippocampus, most excitatory synapses are located on dendritic spines. It has been postulated that the geometry of spines and/or postsynaptic density (PSD) influences synaptic efficiency and may contribute to the expression of plastic processes such as learning or long-term potentiation (LTP). Based on three-dimensional reconstructions of dentate granule cell dendrites from serial electron micrographs, we have measured head dimensions, neck cross-sectional areas, neck length, and PSD area and form of 115 spines of dentate granule cells in the medial perforant path termination zone. All dimensions showed a large variability, with up to 100-fold differences in values. A calculated diffusion index for transport of molecules through the reconstructed neck varied over a 100-fold range. The neck and head dimensions were moderately positively correlated, whereas the PSD area was strongly correlated with head volume. Distribution histograms and scatter plots of various spine dimensions did not reveal any systematic clustering, suggesting that there is a continuum of spine geometries rather than distinct classes for granule cell dendritic spines in the middle molecular layer. Transversely (n = 13) and longitudinally (n = 27) sectioned dendrites had mean spine densities of 2.66 and 1.01 spines/microns, respectively, uncorrected for so-called hidden spines. Bifurcating spines made up 2.1% of the total spine number in transversely and 2.3% in longitudinally sectioned dendrites. The twin spine heads never shared the same presynaptic bouton. Fenestrated or split PSDs shared the same presynaptic element in all but two cases, arguing against PSD division as an intermediate step in synapse formation.

Expression of the neural adhesion molecule L1 in the deafferented dentate gyrus

Expression of the neural adhesion molecule L1 and its potential involvement in axonal sprouting were examined in the deafferented rat dentate gyrus. We focused on the dentate gyrus because of its well-defined cytoarchitecture and well-characterized neuronal degeneration and sprouting response following entorhinal cortex lesions. In the molecular layer of the dentate gyrus, a trilaminar staining pattern was observed, with the middle molecular layer exhibiting slightly denser immunolabeling compared to both inner and outer molecular layers. Two to 12 days after a unilateral entorhinal cortex lesion, a progressive loss of L1 immunolabeling was noted in the ipsilateral middle and outer molecular layers, followed by a substantial reappearance of immunostaining 65 days after lesion incidence. The width of the immunostained ipsilateral inner molecular layer revealed a progressive widening and by postlesion day 65 occupied about 50% of the total width of the molecular layer. Immunoelectron microscopy localized L1 to the surface of unmyelinated axons in both normal and deafferented dentate gyrus. In situ hybridization revealed L1 messenger RNA confined to neurons throughout the hippocampal formation, but did not indicate changes in L1 messenger RNA levels in the hippocampus, dentate gyrus, entorhinal cortex or basal forebrain in response to unilateral entorhinal cortex lesions. Changes in L1 immunolabeling in the deafferented dentate gyrus corresponded in a spatial and temporal manner to changes of the synaptic marker synaptophysin and axonal marker phosphorylated tau. Results of the present study are most consistent with the view that L1 is expressed on reinnervating fibers after they make synaptic contacts with other structures. Thus, L1 appears to be involved in the maturation and stabilization of reinnervating fibers and consequently may play an important role in the repair process of the lesioned adult CNS.

Alterations in apolipoprotein E expression during aging and neurodegeneration

Apolipoprotein E (apoE) is a 34 kDa protein that plays an important role in cholesterol transport, uptake and redistribution. Within the nervous system, apoE might be involved in maintaining synaptic integrity after injury and during aging. ApoE might help maintain the integrity of the synaptodendritic complex by several different mechanisms. Among them, recent studies have suggested that apoE: (1) stabilizes the neuronal cytoskeleton; (2) plays an important role in transporting esterified cholesterol to neurons undergoing reinnervation where it is taken up by the low density lipoprotein receptor-related protein pathway and used as a precursor for the synthesis of new synaptic terminals; (3) regulates interactions between neurons and the extracellular matrix (e.g. laminin); and (4) regulates levels of intracellular calcium. The main objective of the manuscript is to review the current progress in understanding the functions of apoE in the nervous system and how malfunctioning of this molecule might result in neurodegenerative disorders such as Alzheimer's disease.

Effects of entorhinal cortex lesion on learning behavior and on hippocampus in the rat

The initial stage of Alzheimer's disease is characterized by a neuropathological change in the entorhinal cortex. In a previous study it was shown that rats with excitotoxic lesion of entorhinal cortex showed an impaired acquisition of passive and active avoidance responses. In this study a rat with excitotoxic lesion of the entorhinal cortex was tested for 'more operant' behavioral learning (i.e., positive reinforcement operant learning). The hippocampus was also examined histologically as acetylcholinesterase-stained sections, and as synaptophysin immunostained sections and examined biochemically by liquid chromatography. Eight weeks after operation, the bilateral entorhinal cortex lesioned rats showed an impaired acquisition of positive reinforcement operant learning. The lesioned side of unilateral entorhinal cortex lesioned rats showed a decrease of acetylcholinesterase-positive fibers in the CA3, the dentate gyrus, and of synaptophysin-positive substances in the CA3. Biochemical study showed a decreased level of acetylcholine in the CA3, and in the dentate gyrus. The histological and biochemical findings are interpreted as indicating that the entorhinal cortex of the rat provides the major extrinsic synaptic input to the hippocampal formation via the circuit which serves as a relay passage through the dentate gyrus and via direct projections into the hippocampus. Behavioral findings confirmed the importance of the entorhinal cortex in memory acquisition and indicated that rats with a partial neuronal loss in the entorhinal cortex may be a useful model for the memory disturbance of Alzheimer's disease.

Long-lasting transneuronal changes in rat dentate granule cell dendrites after entorhinal cortex lesion. A combined intracellular injection and electron microscopy study

Following entorhinal cortex lesion, inhibitory hippocampal neurons show a persistent rarefication of those dendrites formally receiving entorhinal input. Physiological data indicate a long lasting disequilibrium of inhibition and excitation in the de-entorhinated hippocampus. We analyzed the intracellularly-stained dendritic tree of de-entorhinated excitatory rat granule cells. Granule cells of controls and animals surviving 2, 8, 60 and 270 days after unilateral entorhinal cortex lesion were impaled. Dendrites of control cells were of typical shape, traced to the hippocampal fissure and a complete dye filling of dendrites was ascertained by EM-analysis. Conversely, 60 and 270 days following lesioning, dendrites were only rarely seen to extend into the outer portions of the molecular layer and the dendritic architecture became significantly rarefied. Sixty days post-lesion, intracellularly filled dendrites extending to the middle molecular layer were surrounded by cell clusters resembling glia. Some of these contained the neuronally applied dye, suggesting a close association of the cytosolic compartments with the altered dendrites. These observed alterations exceed the process of sprouting and de novo synaptogenesis of remaining afference for long periods of time. The dendritic morphology of both inhibitory and excitatory neurons seems to require specific input from the entorhinal cortex. Moreover, sprouting of remaining afferents is apparently not sufficient to compensate for this loss of input.